A spark plug for use in an internal combustion engine such as an automotive engine includes a center electrode extending in an axis line direction, for example, an insulator provided on an outside of the center electrode, a cylindrical metal shell provided on an outside of the insulator and a ground electrode joined to a front end portion of the metal shell at a proximal end portion thereof. In general, the ground electrode is bent so that a distal end portion thereof faces a front end portion of the center electrode when the ground electrode is disposed as designed, whereby a spark discharge gap is formed between the front end portion of the center electrode and the distal end portion of the ground electrode for a spark to be discharged in the spark discharge gap defined almost in an axis line of the spark plug. Here, in the case of the spark discharge gap being too large, there is a fear that an increase in required voltage is called for, whereas in the case of the spark discharge gap being too small, there is a fear that a decrease in flame propagation properties is called for. Because of this, the spark discharge gap needs to be adjusted to an appropriate magnitude.
As a method for adjusting the magnitude of a spark discharge gap, there is known a method in which after a bending spacer is disposed so as to face a front end face of a center electrode, by use of a bending punch of which an abutment portion with a ground electrode contact is made into an inclined surface, a preliminary bending step of pressing the ground electrode against the bending spacer is carried out, and thereafter, after the bending spacer is removed, by use of a bending punch having a plat abutment surface, the ground electrode is bent to a predetermined position (refer to Patent Document 1, for example).
Here, in a so-called spark plug of longitudinal discharge type in which a spark is discharged almost in the axis line direction as has been described above, it is possible to adjust the magnitude of the spark discharge gap by use of the aforesaid technique. In recent years, however, with a view to realizing an increase in durability, there have been proposed a so-called spark plug of transverse discharge type in which a distal end portion of a ground electrode is disposed so as to face a side of a center electrode and a so-called spark plug of oblique discharge type in which a distal end portion of a ground electrode is disposed so as to face a front end edge portion of a center electrode. In these types of spark plugs, the magnitude of the spark discharge gap is determined by not only a magnitude thereof defined in the axis line direction but also a magnitude thereof defined in a direction perpendicular to the axis line. Consequently, in the case of adopting the aforesaid technique in which only the magnitude of the spark discharge gap defined in the axis line direction can be adjusted, there is a fear that the magnitude of the spark discharge gap cannot be adjusted with good accuracy.
Then, As shown in FIG. 8, a method is considered in which a magnitude of a spark discharge gap 33 defined in a direction perpendicular to an axis line CL1 of a spark plug is adjusted by use of a gap adjusting unit 83 which moves a ground electrode 27 relative to a center electrode 5 so that the ground electrode 27 moves toward or away from the center electrode 5.